RSS is a syndication format. Syndication is a process by which a web site is able to share information, such as articles, with other web sites for reuse and integration with other material.
In the world of data processing the way different applications and data storage systems communicate with one another is through XML. XML stands for eXtensive Markup Language and is a language that allows information and services to be encoded with meaningful structure and semantics that computers and humans can understand.
Easy to see that XML seems to be the natural choice for site syndication. The problem is deciding on the uinfied standard for “meaningful structure” of the syndicated content. Right now there are two main standards - RSS and Atom.
RSS is actually 9 different standards published by different organizations with the most recent and generally preferred being RSS 2.0.
So what does it all mean to site owners.
As long as you syndicate the content aiming at desktop users you can do it in any format you want. Feed reading software will accept all formats even if they are reasonably malformatted.
Problems start if you want to syndicate your content for other webmasters or use syndicated content from other sites. Since there are several RSS formats you will have to set up feed generators and parsers for all relatively popular formats.